What should be the baseline when calculating excess mortality? New approaches suggest that we have underestimated the impact of the COVID-19 pandemic and previous winter peaks
Excess mortality has been used to measure the impact of COVID-19 over time and across countries. But what
baseline should be chosen? We propose two novel approaches: an alternative retrospective baseline derived from
the lowest weekly death rates achieved in previous years and a within-year baseline based on the average of the
13 lowest weekly death rates within the same year. These baselines express normative levels of the lowest
feasible target death rates. The excess death rates calculated from these baselines are not distorted by past
mortality peaks and do not treat non-pandemic winter mortality excesses as inevitable.
We obtained weekly series for 35 industrialized countries from the Human Mortality Database for 2000–2020.
Observed, baseline and excess mortalities were measured by age-standardized death rates. We assessed weekly
and annual excess death rates driven by the COVID-19 pandemic in 2020 and those related to seasonal respi-
ratory infections in earlier years. There was a distinct geographic pattern with high excess death rates in Eastern
Europe followed by parts of the UK, and countries of Southern and Western Europe. Some Asia-Pacific and
Scandinavian countries experienced lower excess mortality. In 2020 and earlier years, the alternative retro-
spective and the within-year excess mortality figures were higher than estimates based on conventional metrics.
While the latter were typically negative or close to zero in years without extraordinary epidemics, the alternative
estimates were substantial. Cumulation of this "usual" excess over 2–3 years results in human losses comparable
to those caused by COVID-19.
Challenging the view that non-pandemic seasonal winter mortality is inevitable would focus attention on
reducing premature mortality in many countries. As SARS-CoV-2 is unlikely to be the last respiratory pathogen
with the potential to cause a pandemic, such measures would also strengthen global resilience in the face of
similar threats in the future.